1. Field of the Invention
The present invention relates generally to nuclear reactor facilities, and more particularly to a refueling system for use within a liquid metal fast breeder reactor (LMFBR) wherein the refueling system facilitates the reduction in the diametrical extent or size of the closure head large plug so as to be the same as that of the core barrel, while additionally facilitating the transfer of fuel at a station which is located externally of the core barrel yet internally of the reactor vessel, all of the aforenoted also being accomplished without deleteriously affecting the structural integrity of the upper internals structure (UIS) of the facility.
2. Description of the Prior Art
A nuclear reactor produces heat as a result of the fission of nuclear material which is disposed within fuel rods, the fuel rods being secured together so as to define fuel assemblies. The fuel assemblies define the nuclear reactor core, and the core is disposed within a reactor or pressure vessel. In commercial nuclear reactor facilities, the heat produced by means of the fission process is used to generate electricity. Accordingly, conventional facilities usually comprise one or more primary flow and heat transfer loops, and a corresponding number of secondary flow and heat transfer loops to which conventional steam turbines and electrical generators are fluidically connected. A typical energy conversion process for such commercial nuclear reactor facilities would therefore comprise the transfer of heat from the nuclear core to the primary coolant flow system, from the primary coolant flow system to the secondary coolant flow system, and finally from the secondary coolant flow system to the steam turbines, and the electrical generators operatively connected thereto, from which the electricity is ultimately generated.
In a liquid cooled nuclear reactor, such as, for example, a liquid metal-cooled fast breeder reactor (LMFBR), a reactor coolant, such as, for example, liquid sodium, is circulated through the primary coolant flow system which typically comprises the nuclear core, a heat exchanger, and a circulating pump. In nuclear reactors having more than one primary coolant flow loop within the primary coolant flow system, the nuclear core and the reactor pressure vessel, within which the nuclear core is disposed, are connected in common to each of the primary coolant flow loops. The heat generated by means of the nuclear core is thus removed by means of the reactor coolant which is conducted into the reactor vessel and through the reactor core. The heated reactor coolant then exits from the nuclear core and the reactor vessel so as to flow through the heat exchangers which serve to transfer the heat to the secondary flow system associated therewith. The cooled reactor coolant then exits from the heat exchangers and is recirculated back to the reactor pressure vessel by means of the circulating pump, whereby the aforenoted flow cycle is repeated.
The nuclear reactor pressure vessel is sealed at the top portion thereof by means of a cover or closure head, and in fast-neutron energy reactors, such as, for example, a liquid metal-cooled breeder reactor, it is imperative that the closure head include one or more rotatable structural members or plugs. By suitable rotation of these plugs, it is possible for the instrumentation, control, and handling equipment mounted upon or operatively associated with the plugs to be selectively positioned above all desired locations of the nuclear core. In this manner, it is possible to achieve underthe-head or under-the-plug refueling, or in other words, refueling of the nuclear core while the closure head is maintained in its sealed mode atop the pressure vessel and core. This is of course mandatory in connection with liquid metal fast breeder reactors, as such is not in connection with, for example, pressurized water reactors, in view of the explosive instability of the liquid metal coolant in the presence of an ordinary atmospheric environment as would be the case should the closure head be removed for the refueling operation.
In liquid metal-cooled fast breeder reactors, it is also required that a positive top core holddown structure be provided in order to maintain the fuel assemblies in their core positions during reactor operations, or in the unlikely event of a disruptive core accident. The core holddown structure, and the upper internals of which it is an integral part, also function to guide and maintain the alignment of the various control mechanisms and instrumentations during, for example, reactor shutdown refueling operations, as well as for protecting and housing the control rods during reactor operational periods. The core holddown structure is thus positioned atop the nuclear core during normal reactor operations, however, the same must also provide accessibility to the core fuel assemblies during refueling operations of the core by the fuel handling equipment. Consequently, some desirable arrangement or dispositional scheme for the upper internals structure (UIS) must be developed whereby the various operational goals of the facility, both during normal operation and refueling, can be advantageously achieved.
The prior art attempted to resolve the aforenoted, apparently relatively diverse dispositions of the UIS within the normal operational and refueling modes of the reactor, and thereby effectively achieve the operational goals of the reactor both during the normal operations and refueling modes, by mounting the core holddown structure and its UIS upon a small plug rotatably disposed upon a larger plug. During normal reactor operations, the core holddown structure and the UIS is seated atop the nuclear core so as to completely cover the same. When refueling operations are to be commenced, however, the large plug is rotated so as to in turn rotate the small plug and the attached UIS out of position atop the nuclear core and simultaneously position a suitable refueling machine, attached to the underside of the large plug, over the core for performance of the refueling operations. As may therefore be readily appreciated, in order to accommodate that portion of the large plug, upon which the small plug and the UIS are disposed, within the reactor pressure vessel and outside of the normal boundary or shadow of the core barrel when the large plug has been rotated to its refueling mode position, the diametrical extent of the reactor pressure vessel had to be approximately twice that of the core barrel. In view of the well-known fact that the size of the reactor pressure vessel directly affects the size of the nuclear steam supply system (NSSS) containment structures or nuclear plant buildings, which of course therefore determine nuclear plant capital expenditures, the aforenoted structural arrangement or layout of the refueling system has not been a commercially acceptable or viable means of accomplishing the refueling operations within a liquid metal-cooled fast breeder reactor.
Another refueling system structural arrangement developed within the prior art involved the disposition of the core holddown structure upon the small plug in an eccentric manner whereby the core holddown structure could be rotated away from its normal position atop the nuclear core by means of rotation of the small plug. Such a structural arrangement, however, nevertheless required additional space to be provided about the periphery of the nuclear core and within the reactor pressure vessel in order to accommodate the disposition of the holddown structure when refueling operations are being performed. Consequently, the diametrical extent of the large plug was still approximately twice that of the core barrel. In addition, within this particular structural arrangement of the various system components, the UIS was split into at least two separate parts or portions, a first portion being operatively mounted connected to the small plug while the second portion was operatively connected to the large plug. Such a structural arrangement, however, was deemed undesirable from the viewpoint of efficiently carrying out the refueling operations of the plant because it was often difficult to properly align the two different UIS sections relative to each other as well as relative to the nuclear core. Still further, a split UIS is undesirable from the viewpoint of adequately withstanding earthquake shock loads because the structural integrity of a split UIS is substantially less than that of a single or integral, one-piece UIS. While further improved or modified nuclear facilities were in fact able to structurally arrange the large and small plug, and UIS, components so as to substantially reduce the size of the closure head and the diametrical extent of the large plug relative to the diametrical extent of the core barrel, these advanced facilities neverthless still employed split UIS components which of course exhibited the aforenoted unacceptable operational and structural drawbacks. In addition, no provisions were made in connection with these refueling systems for performing the refueling operations within the reactor pressure vessel. Consequently, the refueling operations are performed externally of the reactor pressure vessel which necessitates the disposition of the fuel assemblies within massive shielded refueling machines. As may therefore be readily appreciated, such machines, as well as the special handling equipment required in conjunction therewith, were not in furtherance of the economic cost-effective goals of state-of-the-art nuclear reactor plant facilities.
A last type of conventional nuclear reactor facility refueling system likewise sought to achieve the various aforenoted operational goals characteristic of the normal operational and refueling modes of the facility, while nevertheless seeking to resolve the various aforenoted operational, structural, and economic drawbacks of other conventional systems, through the provision of a large rotary plug system wherein the refueling handling machine was mounted upon the underside of the large rotatable plug so as to be radially movable relative thereto within a radially extending slot defined within the system's UIS. Such a system, however, has likewise proven to be less than entirely satisfactory in view of the fact that a slotted UIS does not exhibit the degree of structural integrity that is desired or required in order to permit reactor facilities to adequately withstand severe shock loading or forces as may be encountered during, for example, earthquake phenomena. In addition, the provision of the radially oriented slot within the UIS for the traversal of the refueling handling machine means that the machine must actually be maneuvered between the control rod mechanisms of the reactor which is very difficult and time-consuming in view of the fact that care must be taken when moving the handling machine within the congested vicinity of the control rod mechanisms.
Accordingly, it is an object of the present invention to provide a new and improved nuclear reactor refueling system.
Another object of the present invention is to provide a new and improved nuclear reactor refueling system which is particularly characteristic of a liquid metal-cooled fast breeder reactor (LMFBR).
Still another object of the present invention is to provide a new and improved nuclear reactor refueling system which overcomes the various operational drawbacks characteristic of conventional or prior art liquid metal-cooled fast breeder reactors.
Yet another object of the present invention is to provide a new and improved nuclear reactor refueling system for a liquid metal-cooled fast breeder reactor which is especially cost-effective.
Still yet another object of the present invention is to provide a new and improved nuclear reactor refueling system for a liquid metal-cooled fast breeder reactor which facilitates the reduction in the diametrical extent or size of the closure head large plug so as to be the same as that of the core barrel.
Yet still another object of the present invention is to provide a new and improved nuclear reactor refueling system for a liquid metal-cooled fast breeder reactor which facilitates the transfer of the reactor core fuel assemblies at a station located externally of the core barrel yet physically within the reactor pressure vessel.
A further object of the present invention is to provide a new and improved nuclear reactor refueling system for a liquid metal-cooled fast breeder reactor wherein the upper internals structure (UIS) operatively associated with the reactor core has a unique structural arrangement.
A yet further object of the present invention is to provide a new and improved nuclear reactor refueling system for a liquid metal-cooled fast breeder reactor wherein the upper internals structure comprises a single or one-piece integral structure.
A still further object of the present invention is to provide a new and improved nuclear reactor refueling system for a liquid metal-cooled fast breeder reactor wherein the one-piece or integral UIS exhibits an exceedingly high degree of structural integrity.
A yet still further object of the present invention is to provide a new and improved nuclear reactor refueling system for a liquid metal-cooled fast breeder reactor wherein the UIS is capable of being disposed in its core holddown mode during normal reactor operation as well as for facilitating the disposition of the control rods within the reactor core in preparation for shutdown of the reactor for the institution of refueling operations, and in addition, the UIS is capable of being suitably moved relative to the reactor core in order to provide the requisite accessibility to the core fuel assemblies during performance of the refueling operation sequence.
A still yet further object of the present invention is to provide a new and improved nuclear reactor refueling system for a liquid metal-cooled fast breeder reactor wherein the UIS, and the reactor control rods and mechanisms operatively associated therewith, is arranged in a symmetrically balanced array relative to the reactor core.
An additional object of the present invention is to provide a new and improved nuclear reactor refueling system for a liquid metal-cooled fast breeder reactor wherein the refueling system handling machine is capable of performing the refueling operations without encountering any interference with the control rods or their mechanisms.